1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device that is electrically writable, readable, and erasable, and its manufacturing method. In particular, the present invention relates to a structure of a floating gate in the nonvolatile semiconductor memory device.
2. Description of the Related Art
The market is expanding for nonvolatile memories in which data can be electrically rewritten and data can be stored even after the power is turned off. Features of a nonvolatile memory lie in that its structure is similar to that of a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a region capable of accumulating charge for a long period of time is provided over a channel forming region. Such a charge accumulating region is formed over an insulating layer and is also referred to as a floating gate since it is insulated and isolated from the peripheral portion. Since the floating gate is electrically insulated from the peripheral portion by being surrounded by an insulator, there is a feature that charge injected to the floating gate are retained. A gate electrode called a control gate is further provided over the floating gate with an insulating layer interposed therebetween. The control gate is distinguished from the floating gate since predetermined voltage is applied to the control gate in data writing or reading.
In a floating gate type nonvolatile memory having such a structure, data is stored through electrical control of charge injection to and discharge from the floating gate. Specifically, charge is injected to and discharged from the floating gate by application of high voltage between a semiconductor layer in which a channel forming region is formed and a control gate. It is said that Fowler-Nordheim (F-N) type tunnel current (NAND type) or thermoelectrons (NOR type) flow in an insulating layer over the channel forming region at this time. Accordingly, the insulating layer is also referred to as a tunnel insulating layer.
The floating gate type nonvolatile memory is required to have a property, for securing reliability, that charges accumulated in the floating gate are retained for 10 years or more. Accordingly, the tunnel insulating layer is required to be formed to have a high insulating property so that charge does not leak and a thickness which produces tunnel current flow.
In addition, the floating gate formed over the tunnel insulating layer is formed using silicon that is the same semiconductor material as that of the semiconductor layer in which the channel forming region is formed. Specifically, a method for forming the floating gate using polycrystalline silicon has been widely used, for example, a polysilicon film deposited with a thickness of 400 nm (refer to Patent Document 1: Japanese Published Patent Application No. 2000-58685, p 7, FIG. 7)